This invention relates to tunable spin-flip Raman lasers and more specifically to cascaded spin-flip Raman lasers of alloy semiconductors such as mercury cadmium telluride to provide for a relatively large change in frequency for a small change in magnetic field.
A conventional spin-flip Raman laser employs a piece of a semiconductor prepared in the form of a resonant cavity immersed in a magnetic field and cooled to a low temperature, say 15.degree. K. or below. A laser pump focused on the semiconductor causes it to emit coherent radiation the frequency of which differs from that of the laser pump, depending on the magnetic field strength. The frequency can be lower (1st Stokes, 2nd Stokes, 3rd Stokes, etc.) or higher (1st anti-Stokes, 2nd anti-Stokes, etc.).
Spin-flip Raman lasers have a limited tuning range. As the magnetic field is increased, the output frequency increases linearly with field over a first range and then becomes less than linear. The output power from the Raman laser increases to a peak with increase in magnetic field to a given level then decreases as the wavelength increases. With increasing field a second peak may be reached; more than two peaks are possible also. In order to achieve a high conversion efficiency (i.e. output power divided by input power) it is necessary to have the energy gap of the semiconductor nearly equal to but slightly greater than the energy of the pump radiation, a condition known as resonance enhancement. For an arbitrary pump frequency this can be achieved through the use of an alloy semiconductor where the energy gap can be selected to meet the matching requirement. For example, mercury cadmium telluride Hg.sub.1-x Cd.sub.x Te has been successfully employed with x approximately 0.23 to match the CO.sub.2 pump laser wavelengths of 10.26 and 10.60 .mu.m. A mercury cadmium telluride Hg.sub.1-x Cd.sub.x Te spin-flip Raman laser as known in the art is generally described by FIG. 1 in which a CO.sub.2 laser is used as a pump for the Hg.sub.1-x Cd.sub.x Te, where x.apprxeq. 0.23. The semiconductor in the form of a resonant cavity element is placed in a dewar and a cryogenic system is used to cool it to, say, 15.degree. K. or below. An electromagnet has pole faces which straddle the dewar to apply a controllable magnetic field to the Hg.sub.1-x Cd.sub.x Te crystal element. The output wavelength of the spin-flip Raman laser is tuned by changing the magnetic field so that, in effect, the combination becomes a tunable CO.sub.2 laser. It is generally explained that the magnetic field splits the energy states for electrons in the conduction band into "spin up" and "spin down" states. The amount of splitting is proportional to the magnetic field and at very low temperatures substantially all of the electrons are in the lowest (spin up) state. When a photon from the CO.sub.2 laser is scattered by an electron in the lowest (spin up) state the electron gains enough energy to enter the upper (spin down) state. This is known as a spin-flip transition. The photon loses the same amount of energy as is gained by the electron, and the photon emerges from the Hg.sub.1-x Cd.sub.x Te element with a lower energy, i.e., a longer wavelength. The energy loss is approximately proportional to the magnetic field applied to the Hg.sub.1-x Cd.sub.x Te element so the output wavelength of the spin-flip Raman laser is controlled by the magnetic field. In addition to the principal emission line, known as first Stokes, there sometimes are weaker lines known as second Stokes, third Stokes, etc., and other weak lines known as first anti-Stokes, second anti-Stokes, etc. This is generally shown graphically in FIG. 2. The second Stokes, third Stokes, etc. lines shift toward longer wavelengths at rates twice, three times, etc. as the first Stokes. The first anti-Stokes, second anti-Stokes, etc. lines shift toward shorter wavelengths in the same manner. A more complete description of this background information is discussed in a publication "Observation of First Stokes, Second Stokes, and Anti-Stokes Radiation From a Mercury Cadmium Telluride Spin-flip Raman Laser", Paul W. Kruse, Applied Physics Letters Vol. 28, No. 2, pages 90-92, Jan. 15, 1976.
Problems with any conventional spin-flip Raman laser, including those using an alloy semiconductor of a uniform composition, include the following:
1. A large magnetic field is needed to achieve a large change in frequency, necessitating in many cases the use of superconducting magnets.
2. The output power, and therefore the conversion efficiency, exhibits two or more peaks with increasing magnetic field, see FIG. 3.
As the magnetic field is reduced from a value substantially greater than 10kG, the first peak appears at about 6-9kG. Further field reduction requires a second peak to appear at 2-3kG. It is probable that there are other peaks below 2kG, although these have not been experimentally verified. Therefore, at the high fields, well above 10kG, needed to obtain large changes in output frequency or wavelength, the conversion efficiency is extremely low, and decreases with increasing magnetic field. For a very large change in frequency the efficiency may be so low as to be relatively useless. The conversion efficiency is the ratio of output power from the spin-flip Raman laser to input power from the pump, say CO.sub.2, laser.
3. The change in frequency with magnetic field, which is linear at low fields, becomes sublinear at higher fields.
This invention directed to a spin-flip Raman laser with increased tuning range and increased efficiency aims at avoiding the difficulties of the prior art mentioned above through the use of an optically cascaded spin-flip Raman laser. A specific embodiment employs Hg.sub.1-x Cd.sub.x Te. However, other alloy semiconductors, e.g., Pb.sub.x Sn.sub.1-x Te, are also included.